Groups of rats, raised from in utero to 15 weeks of age, each group in a different habitat illuminance, had retinas whose photon- catching abilities varied inversely with the habitat: high photon- catching ability in low illuminance, low ability in high illuminance. The results of this early work showed that, regardless of habitat illuminance (over the range of 3 lux to 400 lux), the retinas of all groups of rats absorbed the same number of photons each day. This phenomenon has been called "photostasis" and the objectives of this proposal are to study photostasis in more detail. To wit: 1) Is the regulation of daily photon-catch plastic? Does the regulation persist if an animal is moved from one illuminance to a different one? 2) What is the time course of the adjustment to a "new" intensity? Does it correspond to the turnover time for rod outersegment renewal? 3) What relative roles do disk synthesis and disk shedding play in the adjustment to a "new" intensity? 4) How does the age of an animal affect the ability to adjust to a "new" intensity? 5) Do pigmented rats also show "photostasis"? The studies will help explain why rod outersegment turnover occurs, they will probably impact upon studies of circadian rhythms, and they will show whether the ideas of "photostasis" generalize to pigmented rats. The techniques to be used include visual pigment biochemistry, light-microscopy, LM autoradiography, and microspectrophotometry of single rat rods.